Merrow are Celtic Merefolk. Apparently, if I’m doing my research correctly, they are always half-fish, but the males have to wear fancy hats to breath under water, and the females wear hair combs.
If they lose their magical headgear, they can’t breath underwater anymore, so they just leave their skin behind and become beautiful spirits.
Also, they use magic songs to lure humans underwater where they enchant them.
Apparently they are the only truly nice folk-monster in Celtic mythology.